We are reviewing the use case with a third-party disclaimer service. The servers are located internally in the Azure subscription, and the emails are routed back and forth via a connector before finally being sent out into the world.
I am therefore referring to this information:
"If I send a message to an external recipient but it routes out to a signature service or on-premises for processing, then comes back into Exchange Online to be sent out to the external recipient, will that get counted twice?
To reduce the risk of bad actors spoofing on-premises systems to send spam, we are currently counting these messages more than once. Our telemetry shows only a relatively small number of tenants are currently exceeding their quota, so it’s highly unlikely this will be an issue for your tenant. That said, we’re investigating alternative ways to prevent this type of spoofing that won’t double-count such messages."
We asked MS Support again and they told us that it was a bug and that everything has now been fixed and it will only be counted once. Can you confirm this?